Treating muscle crossbridges in the presence of ATP with the alkylating agent N-phenylmaleimide (NPM) links phenylmaleimide groups to Cys-707 and Cys-697 on myosin heavy chain and totally inhibits the crossbridges' ability to make force. Force is inhibited by NPM because the crossbridges are prevented from undergoing the weakly-binding to strongly-binding conformational change. Treating crossbridges with fluorodinitrobenzine (FDNB) in the presence of pyrophosphate links a dinitrophenyl to Cys-707 and inhibits force by 90%, but it does so by a different mechanism than NPM since the dintrophenylated crossbrdige is still capable of undergoing the weak-to-strong transition. In terms of attachment and detachment rate constants, the weakly- and strongly- binding crossbridge states of the dinitrophenylated crossbridges are very similar to those of normal crossbridges. Treating the FDNB-treated crossbridges with NPM, and then removing the dinitrophenyl group using dithiothreitol creates a third type of crossbridge, one with a single phenylmaleimide linked to Cys-697. These crossbridges behave similarly to those having phenylmaleimide at both reactive sulfhydryls, suggesting that having just a single phenylmaleimide at Cys-697 is sufficient to prevent the weak-to-strong crossbridge conformational change.